This invention relates to the field of separators for electrochemical cells and more particularly to microporous polyolefin separators for such cells.
Lithium-ion batteries are the preferred power source for most portable electronics devices. This is so because lithium-ion batteries exhibit higher energy density, longer cycle life, and higher operational voltage as compared to nickel cadmium, nickel metal hydride, and other rechargeable battery systems. Lithium-ion batteries also provide advantages when compared to lithium primary batteries in that they generally use a carbon-based anode as opposed to highly reactive metallic lithium. The carbon-based anode functions via the intercalation of lithium ions between graphene sheets. Lithium-ion batteries are typically produced in spiral wound and prismatic configurations in which a separator is sandwiched between anode and cathode ribbons. The pores of the separator are then filled with an ionically conductive electrolyte. Lithium-ion polymer batteries are produced using electrode ribbons of similar type with a gel electrolyte sandwiched between them.
Thus battery separators are an integral part of the operation, performance, and safety of lithium-ion batteries. The principal function of the separator is to prevent electrical conduction (i.e., xe2x80x9cshortsxe2x80x9d) between the anode and the cathode while permitting ionic conduction via the electrolyte. In addition to providing good mechanical properties for winding and low electrical resistivity for device performance, separators play an important role in the overall safety of lithium-ion cells. During a thermal excursion above about 120xc2x0 C., a liquid electrolyte-filled separator is expected to xe2x80x9cshutdownxe2x80x9d so that ionic conduction between the anode and cathode is eliminated. Separator composition is the variable that largely determines the shutdown temperature.
In general, polypropylene separators have a shutdown temperature that is too high ( greater than 150xc2x0 C.) for use in lithium-ion applications, while high density polyethylene (HDPE) separators often do not fully shutdown because of xe2x80x9chole formationxe2x80x9d that results from shrinkage and poor mechanical integrity under pressure and high temperature. Ultrahigh molecular weight polyethylene (UHMWPE) separators are usually transformed into a nonporous, clear film upon shutdown. Another advantage of UHMWPE separators is that they can be modified with linear low density polyethylene (LLDPE) or other forms of polyethylene to manipulate the shutdown temperature without compromising mechanical integrity. Therefore, UHMWPE separators offer desirable safety features for use in liquid electrolyte lithium-ion batteries.
Polyolefin separators are generally prepared using either a xe2x80x9cdryxe2x80x9d or xe2x80x9cwetxe2x80x9d method of manufacture, and which of these methods is employed has a profound effect on the porosity, pore size distribution, and tortuosity of the separator. Separators manufactured using the xe2x80x9cdryxe2x80x9d method generally have a distinct slit-pore microstructure with a pore width of approximately 40 nm. Separators manufactured using the xe2x80x9cwetxe2x80x9d process have interconnected spherical and/or elliptical pores with diameters of 50-100 nm. UHMWPE separators have outstanding mechanical properties and exhibit excellent wettability with organic electrolytes.
One physical property of battery separators that is of special concern to battery manufacturers is separator thickness because there exists a need for improved energy and power density in lithium-ion batteries. These concerns have caused battery manufacturers to consider thinner separators. Most separators have thicknesses of 20-50 micrometers, but advances in lithium-ion battery technology have made it desirable for separator manufacturers to provide separators as thin as 8 micrometers. Separator thickness is largely dependent on the manufacturing process employed.
There are two types of electrolyte system commonly used in lithium-ion batteries. The first type of commonly used electrolyte system is a liquid electrolyte system in which a liquid electrolyte is used to provide sufficient ionic conduction between electrodes that are packaged in a cylindrical or prismatic metal can. Liquid electrolyte systems have the advantage of providing a thin electrolyte-filled separator with excellent conductivity. However, the use of a liquid electrolyte system necessitates the use of a metal can to contain the liquid electrolyte system. The use of metal cans limits a battery manufacturer""s options in terms of battery shape and size.
The second type of commonly used electrolyte system is a gel electrolyte system in which a gel electrolyte is sandwiched between the electrodes. One method of creating a gel-electrolyte system entails heating a mixture of liquid electrolyte and semi-crystalline polymer(s). Cooling the liquid electrolyte/polymer mixture forms a gel. A major advantage of this approach is that the electrolyte has been immobilized, much like water is immobilized in the gelatin polymer used to produce the Jell-O(trademark) gelatin. The gel electrolyte system also enables battery manufacturers to bond the electrodes together and utilize a flexible pouch rather than a metal can to package the battery, resulting in increased form factor options (i.e., battery shape and size). However, a major disadvantage of the gel electrolyte system is its increased thickness (approximately two to three times thicker than the liquid electrolyte system), which leads to reduced energy and power density. Also, the gel electrolyte system suffers from (1) poor mechanical properties, making assembly difficult and increasing the incidence of shorting between the electrodes, (2) a tendency to swell at elevated temperatures, and (3) lower conductivity as a compared to liquid electrolyte systems.
It is thus desirable to create an electrolyte system that maximizes the advantages of both the liquid and gel electrolyte systems while minimizing their disadvantages. One prior art attempt to achieve this objective entailed coating a porous separator with a nonporous gel-forming polymer and then sandwiching the resulting separator between two electrodes. The electrode and separator combination was then placed in a flexible pouch and liquid electrolyte was introduced. The liquid electrolyte wicked into the electrodes and then into the edges of the polymer web, and the electrolyte then diffused throughout the polymer web and the polymer web coating. The flexible pouch was heated under pressure to bond the electrodes together and to form a homogeneous gel electrolyte system.
In a second prior art attempt, a slurry of gel-forming polymer and liquid electrolyte was coated onto a polyolefin web. Thereafter, the paste was heated such that the gel-forming polymer seeped or was drawn into the pores of the polyolefin web. Upon cooling, the gel-forming polymer became a rigid gel electrolyte. Thus the pores in the bulk region of the resultant separator were completely filled with gel electrolyte.
The gel electrolyte systems resulting from these two prior art attempts exhibit sufficient ionic conductivity, flexibility, and thickness to allow the user to implement new battery shapes. However, both procedures suffer from increased manufacturing cycle time and a consequent inefficient manufacturing process as a result of the dramatic increase in time required to achieve uniform electrolyte distribution.
Accordingly, there exists a need for use as a lithium-ion battery separator a microporous, thin, open-cell structure that maximizes electrolyte wetting and filling and exhibits high ionic conductivity.
An object of the present invention is to provide a microporous polymer web for use in lithium-ion batteries that contains passageways which foster expedient and uniform electrolyte distribution, thereby resulting in reduced cycle time and resistivity and increased wettability.
The present invention is a freestanding battery separator comprising a microporous polymer web with passageways that provide overall fluid permeability. The polymer web preferably contains UHMWPE and a gel-forming polymer material. The structure of or the pattern formed by the gel-forming polymer material and wettability of the UHMWPE polymer web result in a reduction of the time required to achieve uniform electrolyte distribution throughout the lithium-ion battery.
In a first embodiment, the gel-forming polymer material is a microporous coating on the UHMWPE web surface. Using a microporous coating facilitates expedited uniform distribution of electrolyte and thereby reduces manufacturing cycle time of the battery. Additionally, the microporous coating allows the separator to be bonded to the electrodes and allows the formation of a hybrid electrolyte system in which both gel and liquid electrolyte co-exist. This hybrid electrolyte system results in a thin (less than 50 micrometers) separator with good flexibility and higher ionic conduction than a single-phase gel electrolyte system.
In a second embodiment, the gel-forming polymer material is incorporated into the UHMWPE web during an extrusion process. During a subsequent extraction process, passageways that provide overall fluid permeability are formed throughout the web. More specifically, the gel-forming polymer material is added to an extruder with UHMWPE and a plasticizer. The mixture is processed at an elevated temperature to yield a thin film. Subsequent removal of the plasticizer by an extraction process results in a microporous freestanding unitary structure in which regions of gel-forming polymer are dispersed throughout the microporous structure.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings.